The invention relates to a method for operating a drive device of a motor vehicle, said drive device comprising at least one combustion engine and at least one electric machine, as well as a dual clutch transmission which can be functionally connected to the combustion engine and which comprises a first sub-transmission, a first clutch being associated therewith, and a second sub-transmission, a second clutch and the electric machine being associated therewith. In a purely electrical driving operation, in particular for generating a forward torque for the motor vehicle or, respectively, to the driving wheels of said motor vehicle, the electric machine is operated in a motorized manner, a driving gear is engaged in the second sub-transmission and both clutches are disengaged.
The invention further relates to a device for operating a drive device of a motor vehicle, in particular for carrying out the method described above, said drive device comprising at least one combustion engine and at least one electric machine, as well as a dual clutch transmission which can be functionally connected to the combustion engine and which comprises a first sub-transmission, a first clutch being associated therewith, and a second sub-transmission, a second clutch and the electric machine being associated therewith. The device comprises means for controlling the drive device and said means control said drive device for a purely electrical driving operation in such a manner that the electrical machine is operated in a motorized manner, a driving gear is engaged in the second sub-transmission and both clutches are disengaged.
Methods of the kind mentioned above are known from the prior art. In the case of so-called hybrid drive devices, a combustion engine as well as an electric machine can be used to drive the motor vehicle. The drive device can thereby be operated such that the combustion engine and the electric machine apply a collective torque to the drive wheels of the motor vehicle, or a driving operation purely in the combustion engine mode or a purely electrical driving operation occurs. In the case of a purely electrical driving operation, both clutches to the combustion engine are disengaged; and a gear is engaged in the sub-transmission with which the electric machine is associated in order to direct the torque generated by the electric machine to the driving wheels. In the process, the electric machine is supplied with electrical energy from an electrical storage unit of the drive device. If the charging state of the electrical storage unit reaches a critical state for further operation of the electric machine, it is necessary to recharge the electrical storage unit by means of said electric machine being operated in a generating manner. To this end, the drive torque from the other drive unit, i.e. from the combustion engine, must be provided. Had the combustion engine been previously switched off, for example in order to reduce fuel consumption, said combustion engine must be restarted. If said combustion engine is not provided with a separate starter, the starting of said combustion engine generally occurs with the aid of the electric machine.
In order to solve this problem, the unexamined German patent application DE 10 2007 050 659 A1 discloses a method in which, for starting the combustion engine, the second clutch is kept disengaged and the clutch associated with the first sub-transmission is engaged at least to the point where said clutch transmits a torque required for starting the combustion engine.
A method is further known from the international WIPO patent application WO 2010/009943, in which in order to start the combustion engine from an electric driving state, the second clutch is engaged when the vehicle is stationary in order to additionally power said combustion engine by means of the electric machine, wherein a gear is engaged in the first sub-transmission having a higher gear ratio than the engaged gear of the second sub-transmission.
When starting the combustion engine according to the unexamined publication of the German patent application mentioned above, a minimum vehicle speed is required in order to start the combustion engine, whereas it is necessary for the vehicle to be stationary in the method according to the unexamined international WIPO patent application.
The known methods cannot be used for situations in which a slow crawl of the vehicle at low speeds is required, in particular for longer periods of time. Such a situation arises, for example, in traffic jams.